Extruding vertices with x mirror
X Mirror only works when manipulating vertexes, edges and faces in position scale and rotation.

For any work such as extruding etc, it breaks out of the mirroring tool and will no longer be symetrically affected.

To continue working with a mirrored object, you will have to delete all of the vertexes on one side of the model and add a mirror modifier.

Select the view all vertexes without clipping button:



Select all the vertexes on either the left or right of an object, but avoid selecting the vertexes down the centre of the object, (you'll need these for flipping it)



Delete these vertexes and add a mirror modifier:



You'll now have an object that is perfectly mirrored. If it doesn't look right, make sure you've got the right axis selected in the mirror modifier.